Incinerator rotary kilns have an outside metallic shell, usually steel, the inside of which is fully covered by a thick ceramic wall, usually in the form of fire bricks having a total thickness exceeding in many cases 25 cm. These kilns operate at a temperature in the range of about 1,800.degree. to 2,200 .degree. F. The ceramic walls, however, are very vulnerable to erosion and corrosion, due to the hostile conditions created by the nature of incinerated materials and high temperatures, especially, if alkali metals are present. If the viscosity of slag in the kiln is adequately high, it may form a rather thick viscous coating on the ceramic and thus protect it from the hostile environment. However, when the viscosity of the slag is very low, the slag contributes to the erosion and corrosion of the ceramic, both chemically because it serves as a solvent and mechanically, as it allows foreign big pieces of abrasive material to act against the ceramic walls. If the slag is viscous to the point of becoming substantially solid, or if it has never been formed as liquid, it becomes ineffective in promoting combustion of organic matter, and also in capturing toxic heavy metals. Therefore, it is imperative that the viscosity of the slag is very carefully monitored and controlled within a range of values. Thus, one of the objects of this invention is to detect and correct the viscosity of the slag in rotary incinerating kilns. In addition, this invention provides a novel method of measuring viscosity of liquids in general.